The present invention relates broadly to an eddy current probe apparatus, and in particular to a ferromagnetic eddy current probe apparatus.
Nondestructive test methods are used to inspect the structural integrity of critical structural members, particularly in aircraft. Eddy current testing has proved to be invaluable in detecting metal flaws in fastener holes when the cracks are minute-size, less than 0.010 inch in depth, and 0.010 inches long axially. At present, it is the only practical method for reliably detecting small, tight cracks with a minimum of hole and surface preparation prior to testing. Eddy current techniques require the use of probes to initiate eddy currents into a test specimen or structural member. This is accomplished through the use of eddy current probe in which an alternating current at a suitable frequency is applied to a test coil that is located at the probe tip. The magnetic field in the test coil induces eddy currents into the structural part which is in contact with or in close proximity to the eddy current probe.
Prior to the development of automatic eddy current scanning at McClellan AFB by this inventor, the standard eddy current technique for detecting fatigue cracks in fastener holes was the hand-scanning method. This involved rotating a hand-held eddy current probe 360 degrees by hand thru a fastener hole while an inspector observed needle movement of the eddy current instrument. Due to the many negative factors associated with performing hand inspection in general, and the difficulty of hand-probe inspection of fastener holes in particular, more than 200 Automatic Eddy Current Flaw Detector Systems are in use by Air Force NDI personnel today.
The present invention relates to a special fastener hole probe with increased sensitivity which cannot become deformed while rotating at any speed. The present probes which are made of nylon or Delrin can become deformed when operated in an automatic detection system that rotates the test probe. Therefore, it may be seen that the new special probe greatly improves the reliability of the Automatic Eddy Current Flaw Detection Systems which is in use throughout the Air Force.